Has n, :through

post.rb

post '/races/new' do
  # params = {"cars" => [ {"car_id" => 1, "duration" => 30},  {"car_id" => 2, "duration" => 40} ] }

  race = Race.new


  # Assuming that "car_id" is the same as the DB ID
  params["cars"].each do |car|
    # Here we are making a new "Racing" for each car listed in the params. See that I'm using ":car_id" and not ":car" which I showed you above. In the above ":car" accepted an entire Car model, but it can also accept just the ID of a Car by doing "
    race.racings << Racing.new(:car_id => car["car_id"], :duration => car["duration"])
  end

  # If your rfid ID isn't the same as the car.id do this
  params["cars"].each do |car|
    car = Car.first(:rfid => car["rfid"])
    race.racings << Racing.new(:car => car, :duration => car["duration"])
  end

  race.save
end

races.rb

## DataMapper Models

# Below is the example for "has n, :through" from the DataMapper page on
# associations here: http://datamapper.org/docs/associations.html

class Car 
  include DataMapper::Resource

  has n, :racings
  has n, :races, :through => :racings
end

class Race
  include DataMapper::Resource

  property :id, Serial, :key => true

  has n, :racings
  has n, :cars, :through => :racings
end

class Racing
  include DataMapper::Resource

  property :id, Serial, :key => true
  property :duration, Integer # time duration of a race in whole numbers

  belongs_to :race, :key => true
  belongs_to :car, :key => true
end

## Creating a Race etc.

car1 = Car.create
car2 = Car.create
car3 = Car.create

# Those three cars race
race = Race.new
race.racings # An empty array
race.racings << Racing.new(:car => car1, :duration => 45)
race.racings << Racing.new(:car => car2, :duration => 23)
race.racings << Racing.new(:car => car3, :duration => 31)

race.save
race.racings # An array of 3 racing objects
race.cars # An array of 3 cars that were in the race